Fluids exhibit a measurable property known as viscosity, a term which may be broadly defined as the internal friction or molecular attraction of a given material which manifests itself in resistance to flow. It is measured in liquids by standard test procedures and is usually expressed in poise or centipoise (cP) at a specified temperature, although in oil technology it is sometimes expressed in seconds required for a given volume to flow through a specified orifice at a definite temperature. The viscosity of a fluid is an indication of a number of behavior patterns of the liquid at a given temperature including pumping characteristics, rate of flow, wetting properties, and a tendency or capacity to suspend an insoluble particulate material.
There are a number of industrial operations wherein it is desirable to employ viscous aqueous solutions. For example, viscous aqueous solutions are employed to fracture subterranean formations penetrated by a borehole for increasing the production of petroleum fluids, namely crude oil and natural gas. Viscous aqueous solutions are also employed in the secondary recovery of oil from oil bearing subterranean formations by fluid drive processes.
It is common practice to treat subterranean formations to increase the permeability or conductivity of such formations by procedures that are identified generally as fracturing processes. For example, it is a conventional practice to hydraulically fracture a well bore in order to produce one or more cracks or “fractures” in the surrounding formation by mechanical breakdown of the formation. Fracturing may be carried out in wells that are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing, or other stimulation procedures, are production wells completed in oil and/or gas containing formations. However, disposal wells and injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations.
Hydraulic fracturing is accomplished by injecting a well treatment fluid into the well bore and applying sufficient pressure on the well treatment fluid to cause the formation to break down with the attendant production of one or more fractures. The well treatment fluid may include a suspended proppant such as sand or other particulate material. The proppant may be deposited in the fractures and functions to hold the fractures open after the pressure is released and the well treatment fluid flows back to the surface. The well treatment fluid must have a sufficiently high viscosity to retain the proppant in suspension or at least to reduce the tendency of the proppant to settle out as it flows along the created fracture. Viscosifiers, such as a polysaccharide or a polyacrylamide, are often used to gel the well treatment fluid to provide the high viscosity needed to retain the proppant in suspension.
After the high viscosity well treatment fluid has been pumped into the well bore and fracturing of the formation and deposition of the proppant has occurred, it is desirable to remove the fluid from the formation to allow hydrocarbon production through the new fractures. Generally, the removal of the highly viscous well treatment fluid is realized by “breaking” the gel, i.e., converting the well treatment fluid into a low viscosity fluid. Breaking the gelled well treatment fluid has commonly been accomplished by adding a “breaker,” that is, a viscosity-reducing agent, to the well treatment fluid prior to pumping the well treatment fluid into the subterranean formation.
Acrylamide-containing polymers are often added to well treatment fluids in oilfield applications to reduce friction or to modify rheological properties of the fluid. Such polymers may persist in the fluid or formation for an extended period of time. The polymer residue may plug some of the permeability of the rock, hinder recovery of the water used in the well treatment fluid, or reduce the availability of a source of nitrogen that can support the growth of bacteria in the well. In order to facilitate removal of these polymers after the task has been completed, treatments involving oxidizer-based breakers, for example persulfate salts, may be applied. Breakers can be used in this manner to generate smaller polymeric fragments with more benign environmental profile.